1. Field of the Invention
This invention relates to zinc alloy-plated corrosion preventive steel sheets having an organic coating thereon. The organic coated steel sheets have a good corrosion resistance and a good adhesion under forming conditions and thus, have wide utility in various fields of electro painting, second coating and top coating of automobile bodies and home appliances. The invention also relates to a method for making the corrosion preventive steel plates of the type mentioned above.
2. Description of the Prior Art
In the industries of automobiles and home appliances, there has recently been a strong demand for corrosion preventive steel sheets have been recently. Accordingly, hitherto employed coldrolled steel sheets and plated steel sheets tend to be replaced by surface-treated steel sheets having a better corrosion resistance. The surface-treated steel sheets used, for example, as bodies for automobiles are a corrosion preventive steel sheet generally called "zinchro metal". This includes, on a steel sheet, a first layer containing a chromium compound and a second layer of a zinc-rich paint comprising zinc powder and a resin and is hereinafter called zinc-rich corrosion preventive steel sheet. The zinc-rich corrosion preventive steel sheet has a good corrosion resistance and is capable of satisfactorily undergoing electro deposition painting and spot welding which are essentially required for use a steel sheet for automobiles.
However, the zinc-rich corrision preventive steel sheet has the following problems.
(1) In order to ensure satifactory corrosion resistance and electro deposition properties, the zinc powder should be contained in the second layer in amounts not less than 80 wt%, with the great possibility of causing the film to separate as fine powder when the steel sheet is pressed thereby forming defects in the sheet.
(2) In case where the zinc-rich film is painted in a thickness as great as, for example, 10 micrometers or over, the zinc powder and the resin present in large amounts are left as a combustion residue when the steel sheet is subjected to continuous spot welding. This entails contamination of electrode chips with the residue with an increasing frequency of chip dressing, thus resulting in poor workability.
In order to solve the problems involved in the zinc-rich corrosion preventive steel sheet, there have been proposed improved corrosion preventive steel sheets, for example, in Japanese Laid-open Patent Application Nos. 57-108292 and 58-98172. The corrosion preventive steel sheets proposed in these Laid-open Applications have, on the surface thereof, composite organosilicate resin films comprised mainly of resins having a hydroxyl group or groups in the molecule, such as polyvinyl alcohol, acrylic resins, epoxy resins, polyesters and the like, to which silica sol and silane compounds are added, or resin films of the above composite organosilicate resins to which oxyacids of Mo, W, V, Sn, B and the like and salts hereof are added. However, these corrosion preventive steel sheets are still unsatisfactory in the corrosion resistance and involves a problem with respect to film adhesion weldability, cratering resistance and paint finish at the time of forming when applied as a steel sheet for automobiles.